This invention relates to a method and apparatus for dechucking a substrate particularly, although not exclusively, a semi-conductor wafer from an electrostatic chuck.
Electrostatic chucks have been used to hold semiconductor wafers and other materials whilst processing in a vacuum chamber. Electrostatic chucks have been used in some cases in preference to mechanical clamping devices to avoid the need for fingers or the like which may damage the surface of the wafer. However, with the use of electrostatic chucks, it has been found that residual forces holding the wafer to the chuck exist even after the clamping voltage is removed after processing.
U.S. Pat. No. 5,684,669 and U.S. Pat. No. 5,325,261 disclose methods aimed at minimising the residual electrostatic force between the wafer and chuck by applying a voltage, which is typically of the same polarity as (but smaller than) the chucking voltage, during release of the wafer. U.S. Pat. No. 5,684,669 uses the leak rate of gas which seeps through the gap between the wafer and the chuck to measure the residual force indirectly. U.S. Pat. No. 5,325,261 monitors the extent of the residual force indirectly by measuring the chuck capacitance.
U.S. Pat. No. 5,221,450 discloses an electrostatic chucking method in which a chuck is immersed in a plasma without the wafer on it in order to remove the remaining charges before the next wafer is held.
It has been found that there are two main sources of stiction in an electrostatic chuck. The first of these discussed above is the residual electrostatic polarisation, of the dielectric material of the electrostatic chuck itself which causes bound surface charge to bind the wafer by electrostatic attraction. The second source relates to residual free charges which are due to either leakage current through or from the surface of the chuck dielectric or derived from the process itself, which reside on the chuck or the wafer surface. Whilst the first problem is addressed by the methods described in U.S. Pat. No. 5,325,261 and U.S. Pat. No. 5,684,669, it has been found that these methods do not work in an entirely satisfactory manner as they do not address the issue of the second source of stiction before or during wafer removal.
According to a first aspect of the present invention, there is provided a method of dechucking from an electrostatic chuck a substrate held by one or more residual forces to the chuck, the method comprising the steps of:
(a) reducing a residual chucking force due to the electrostatic chuck polarisation;
(b) contacting the chuck with the substrate attached thereto with a plasma for a time sufficient substantially to remove any residual charge from the surface of the substrate and the chuck; and
(c) subsequently to, or simultaneously with, step (b) removing the substrate from the chuck.
Preferably, the chuck with the substrate attached thereto is substantially immersed in the plasma.
The contact with or immersion in the plasma may be carried out in any suitable manner, specific examples being immersion in a driven plasma and immersion in, for example, a downstream plasma. Discharging works, for example, both with a coil discharge and a coil and a platen discharge.
The residual chucking force may be the residual electrostatic polarisation referred to above.
The residual chucking force may be minimised by any method known in the art, in particular the methods disclosed in U.S. Pat. No. 5,325,261 and U.S. Pat. No. 5,684,669, and it is therefore not proposed to discuss these in further great detail here. However, it is preferred that the chucking force is minimised by applying a first voltage to the chuck. In a particular embodiment, the preferred magnitude of the first voltage is determined by ramping the voltage down whilst measuring the extent to which the substrate is held to the chuck. Preferably, when a minimum in the residual chucking force is found, the preferred magnitude of the first voltage is held for a desired period. The extent to which the substrate is held may be measured by, for example, the methods disclosed in U.S. Pat. No. 5,325,261 or U.S. Pat. No. 5,684,669. For example, U.S. Pat, No. 5,325,261 monitors the extent of the residual force by measuring the chuck capacitance, as mentioned above. The value of the capacitance depends on how closely the substrate is held to the chuck. As substrates, particularly in the form of wafers, may have some degree of bowing, the force holding the substrate to the chuck may be counteracted by internal stress in the substrate. As the residual clamping force decreases, the substrate""s natural bow is restored and as this happens the substrate capacitance may decrease. Alternatively, as described in U.S. Pat. No. 5,684,669, the residual force may be monitored by measuring the leak rate of a gas which seeps through the gap between the substrate and the chuck.
The chuck may be of any appropriate form. Thus, it may be flat as disclosed in U.S. Pat. No. 5,325,261. Alternatively, it may have one or more steps on its upper surface adjacent the substrate, as also disclosed in U.S. Pat. No. 5,325,261, for example around the periphery of the upper surface.
It is to be noted that, in the method of the present invention, the substrate is still present on the chuck on immersion in a plasma. It may also be subsequently lifted whilst being immersed in a plasma.
Any suitable plasma can be used for the contact or immersion. For example, argon or another inert gas may be present to prevent further etching of the substrate. SF6 and/or other electronegative gases (e.g. oxygen) may be present in the plasma and may improve charge removal. Any suitable power which sustains a plasma may be used, but typical examples are within the range 50 W to 800 W.
The time sufficient to remove the residual charge may be determined by monitoring the capacitance of the chuck due to the proximity of the substrate. Any time may be applicable, but it is typically in the range of between about one second and one minute. It has been found that, in one embodiment, the time sufficient substantially to remove the residual charge is typically about one second.
Preferably, a second voltage is applied to the chuck during contact of the chuck and substrate with the plasma. Whilst any suitable voltage may be used (including a zero voltage) for the second voltage, in a preferred embodiment it is the same as the first voltage.
The applicant has found that reversing steps (a) and (b) of the method of the invention does not lead to efficient removal of the residual charges.
According to a second aspect of the present invention, there is provided an apparatus for performing the method described above comprising:
(a) a chamber within which a substrate held on an electrostatic chuck may be processed;
(b) means for reducing a residual force due to the electrostatic chuck; and
(c) means for providing a plasma to contact the chuck with the substrate attached thereto or being removed therefrom for a time substantially to remove any residual charge from the surface of the substrate and the chuck.
The apparatus may further comprise means for removing the substrate from the chuck. In a preferred embodiment, means may be provided to apply a voltage to the chuck.
Although the invention has been defined above, it is to be understood that it includes any inventive combination of the features set out above or in the following description.